Merc 165 No Spark at Plugs

[Jeklund]

I've recently been gifted a boat from a family member that has been sitting for about 3-4 years. It was running at one point and we took it out prior to putting it into storage. The engine is a Mercruiser 165 Chevy L6 serial number 5390935. The motor cranks but will not spark.

So far I've replaced the coil, distributor cap and rotor, I've cleaned the points as well. I have spark at the points but it will not make it down to the plugs. The only other idea I have is new plug wires but I feel like I would get some sort of spark from one of the wires as they aren't in that bad of condition. I'm at a little bit of a loss right now for what to do next and don't really want to keep throwing parts at the thing.

With the key on the coil gets about 6.5v, one weird thing I noticed is that with the key on the hour meter clicks. It also does it when I jump the coil + terminal directly from the battery with the key off. I've read that maybe that has something to do with the shift interrupter switch so I cleaned the contacts on that as well but I don't really know if that's related.

 

[Fishermark]

You can disconnect the shift interrupt switch temporary to see if that is the problem. It shouldn't be engaged - but if it is then it will ground out the ignition and you will not get a spark.

How are you testing for a spark? Try supplying the full 12 volts to the coil and try to see if you have a spark. Could just be that you didn't clean the points well enough.

 

[achris]

If you have spark across the points gap that's good, shows power is getting to the system.

Remove the coil to distributor cap spark lead from the cap and put a spark plug in the end. Lay the spark plug on the engine and see it you have spark at that plug.

I would also just replace the points and condensor, they are service items anyway.

The hour meter clicking with the key on is correct, that's how the hour meter works. Key on, power to the meter. Same when you jump power to the coil, the power runs back up the line and everything appears the same as turning the key on.

Chris........

 

[Jeklund]

Thank you both for your responses, I will try out both suggestions once the rain lets up here. I'll also look into replacing the points and condensor.

Fishermark the method I was using to test for spark at the plugs was just disconnecting one of the plug wires, placing a new plug in the end and lying it on the engine while cranking and looking for spark.

 

[Jeklund]

Alright, I just went out to give those two things a try. The plug I put in the wire from the coil did spark so I assume that's good news on some level? Disconnecting the shift interrupt switch did not improve anything either, I actually lost the little screw that holds the wires and bracket on so now I'll need to grab a magnet and go hunting for it.

What's my next step know that I have seen a spark at the plug directly from the coil?

 

[stonyloam]

Just a bit about how the points ignition works. Your coil is a step up transformer, low voltage in high voltage out. With the ignition on, when your points close current flows through the primary windings (few turns) of the coil to ground and a electromagnetic field builds up. When your points open the current flow stops and the field collapses and a high (20,000 or so) voltage is generated in the secondary windings (many turns), that high voltage flows out of the center wire of the coil to the center of the distributor cap, through a contact to the rotor, then jumps the gap to the spark plug wire terminal, through the wire to the plug where it jumps the plug gap and ignites the fuel vapors in the cylinder. So your timing has to be correct to allow the jump from the rotor to the plug wire terminal. You need to find the timing mark on the harmonic balancer and put a timing light on it to make sure you are close.

 

[Jeklund]

Just a bit about how the points ignition works. Your coil is a step up transformer, low voltage in high voltage out. With the ignition on, when your points close current flows through the primary windings (few turns) of the coil to ground and a electromagnetic field builds up. When your points open the current flow stops and the field collapses and a high (20,000 or so) voltage is generated in the secondary windings (many turns), that high voltage flows out of the center wire of the coil to the center of the distributor cap, through a contact to the rotor, then jumps the gap to the spark plug wire terminal, through the wire to the plug where it jumps the plug gap and ignites the fuel vapors in the cylinder. So your timing has to be correct to allow the jump from the rotor to the plug wire terminal. You need to find the timing mark on the harmonic balancer and put a timing light on it to make sure you are close.

Thank you for the explanation, apologies if this is a dumb question but is the timing something that could be off after the boat has been sitting? And if it was off significantly could that be the source of my spark problem? I like to consider myself fairly mechanically inclined but the timing on engines is something I don't really understand fully and/or have spent any time doing. I suppose if that's is a potential problem I'll have to look into the process for setting it correctly.

 

[achris]

Timing will not, can not, effect spark quality. Changes in the points WILL effect timing. When you change points, you must check/adjust the timing.

Ok, spark at the end of the coil -> dissy cap lead, no spark at the end of the spark plug lead. So, fault lies in the path, dissy cap -> rotor -> dissy cap -> spark plug lead. You have replaced dissy cap and rotor, leaves the leads, or.... a faulty cap or rotor. Leads do go, but usually not all 6 at the same time. Do you have spark from any of the leads? Also, how 'solid' is the spark you're seeing? Should be fat, blue... Weak yellow is not good.

Chris.....

 

[Jeklund]

The spark I was seeing was not solid by that description. It was yellow/orange. I will also have to check each plug wire tonight and see as I haven't gone through each one.

 

[Maclin]

If you observe it at night you may be able to see where spark is "leaking".

 

[Jeklund]

If you observe it at night you may be able to see where spark is "leaking".

Sorry I don't really follow that. What would I be looking for to check if it's "leaking"?

 

[stonyloam]

I agree with Chris. Anyway timing: your camshaft opens and closes your valves, 4 cycles, 1 intake (piston goes down), 2 compression (up), 3 ignition (down), 4 exhaust (up). Your cam also drives the distributor (the distributor rotates once for every two turns of the crankshaft) in sync with your valves. The distributor has a cam of its own ( 6 lobes in your case) that opens and closes the points. For proper timing the distributor cam opens the points at the top of the compression stroke and sends the spark to the distributor, if timed correctly the rotor is pointed at the correct spark plug wire terminal and the spark travels to the plug. Two terms that you will encounter: dwell and advance. Dwell is the amount of time ( in degrees of rotation) the points are closed allowing the coil to charge. Too short the coil will not fully charge, too long too much current flows and the coil can get hot. Advance: for optimal ignition you want the plug to spark just before (In advance) the top of the compression stroke. You advance the timing by rotating the distributor so that the cam opens the points sooner in the cycle. You advance by turning the distributor in the direction opposite of its rotation. In your case ( think) the dist. Rotates clockwise so you advance by turning the dist. COUNTERCLOCKWISE. One other tern, retard is the opposite of advance.

 

[achris]

The spark I was seeing was not solid by that description. It was yellow/orange. I will also have to check each plug wire tonight and see as I haven't gone through each one.

Weak spark could be, wrong voltage at the coil (that could be anything from a bad key switch to a bad engine harness connector, but let's work on the easier ones first), coil with an internal ballast resistor used in circuit with external ballast resistor, burnt points, faulty condensor, bad HT lead.

I noticed you replaced the coil. Those engine harnesses have a built-in resistive wire. You need to put a coil in that has on it "Use with external resistor"... If you still have the original coil, put it back in, there is a 98.72% chance there's nothing wrong with it.

Chris...

 

[Jeklund]

Weak spark could be, wrong voltage at the coil (that could be anything from a bad key switch to a bad engine harness connector, but let's work on the easier ones first), coil with an internal ballast resistor used in circuit with external ballast resistor, burnt points, faulty condensor, bad HT lead.

I noticed you replaced the coil. Those engine harnesses have a built-in resistive wire. You need to put a coil in that has on it "Use with external resistor"... If you still have the original coil, put it back in, there is a 98.72% chance there's nothing wrong with it.

Chris...

I will try the old one tonight and report back. I definitely could have got the incorrect one and put it in prior to checking the other things because the coil was the first thing I replaced. On the one I purchased, I don't recall any notices on that so that definitely could be the part of the issue now.

 

[Jeklund]

Some great news! I got home today and changed out the coil with the old one and then decided to check the plugs (Probably should have done that as one of the first things). The plugs weren't looking the best so I switched them out with some new ones I had ordered. We threw a little fuel in the carb and she turned right over.

Now here's my next issue, the motor won't stay running and it appears as if it's not getting enough fuel. Over the last few days while cranking it I could definitely smell that it was getting fuel. Today, however, we had it running, the only way to keep it going was to continue to pour a little bit of fuel down the carb. Is that a blatant sign of anything? I'll have to pull out the service manual to start reading up on the fuel system but if anyone has any suggestions on where to start it would be greatly appreciated.

 

[nola mike]

Is that 4 year old gas? That's first culprit, along with checking your fuel filters. Should be at least 2, at the fuel pump and carb entrance.

 

[Jeklund]

The gas is new, we siphoned the tank at the start of this year. Fuel filters should probably be replaced.

 

[achris]

Some great news! I got home today and changed out the coil with the old one and then decided to check the plugs (Probably should have done that as one of the first things). The plugs weren't looking the best so I switched them out with some new ones I had ordered. We threw a little fuel in the carb and she turned right over.

Nice that you have a win... One rule of troubleshooting I use, only change one thing at a time. But hey, you have a spare coil (or do you? ) Was the new coil the wrong one, or is it right, and the plugs were the problem all along? Guess we'll never know.

Fuel problem.... Could be fuel pump not pumping (for any one of many reasons), could be a fuel line problem, could be a filter problem. Or it could be a carb problem, stuck needle, stuck float, gummed up idle jet, gummed up main jet.... etc etc....

I would pull all the lines and replace them, replace the fuel filter and pull the pump and check it out. Once they are confirmed good, if it's still not working, strip and clean the carb....

Chris........

 

[Jeklund]

Ordered the filter a few days ago and just got it in today. The old one was full of red rusty looking crap. I pulled the line from the tank to the pump and blew through to make sure it was clear, it seemed to have a slight block in it but it didn't take much air to blow clear through it.

Also pulled off the metal line at the pump and blew through it to towards the carb, it had some resistance but felt consistent. My thought is its designed that way so it limits the amount of fuel pushing through?

Also pulled the vent line from the pump to carb. it was full of a green sludge that I really couldn't clear. I was able to get it so some air would pass and tried to run the motor to no avail. I'm definitely going to replace that line as it's an obvious problem. I am curious as to whether that is something that could affect the fuel delivery? My understanding is its pretty much there as a backup and you should never really see fuel in it. I am slightly concerned that the green sludge in it flowed back into the pump, maybe that is part of the issue? Also is that line something I can just buy at an auto part store from just a bulk roll? and if so what is it made of?

 

[stonyloam]

That "vent" line is there in case the pump diaphragm ruptures, it dumps leaked fuel into the carburetor rather than allowing it to dump into the bilge.